1. Field of the Invention
The present invention relates to a manufacturing method of a photomask that is preferable to the miniaturization of a pattern of a semiconductor apparatus and a manufacturing method of a semiconductor apparatus using the photomask. In particular, the present invention relates to a manufacturing method of a photomask that optimizes a sub resolution assist feature on a photomask so as to ensure the depth of focus for a formed image of a pattern for circuit formation of the photomask and a manufacturing method of a semiconductor apparatus using the photomask.
2. Description of the Related Art
In accordance with the miniaturization of the device size of a semiconductor apparatus, a resist pattern created by exposure from a photomask pattern is a pattern of approximately half of a wavelength of illumination light used for the exposure, i.e., a sub wavelength pattern in a photo-lithography step serving as one of manufacturing steps of the semiconductor apparatus. Then, in order to resolve the sub wavelength pattern, such a design is performed that an exposure device with high lens-performance having a high numerical aperture is used, a correcting pattern with respect to the optical proximity effect is added to the photomask pattern, and an off-axis illumination method serving as one of photomask illuminations is used.
Upon using the off-axis method to the photomask illumination method, an angle (identical to an angle of the 0-th-order diffraction light) of illumination light is set so that 0-th-order diffraction light and first-order diffraction light from a high-density portion of the photomask is incident on a lens. Then, with the off-axis method, the 0-th order diffraction light and the first-order diffraction light necessary for forming a pattern image can be incident even on a lens having a numerical aperture that is not greatly high.
Therefore, if the first-order diffraction light is greatly diffracted by the pattern portion with high density on the photomask, the resolution of the formed pattern image is improved, thereby obtaining a required depth of focus. Incidentally, with the necessary depth of focus, a pattern can be preferably formed even in consideration of the change in the best focusing position based on concaved and projected portions on the surface of a semiconductor apparatus or the focusing precision of the semiconductor apparatus.
A technology is proposed to ensure the necessary depth of focus even for a formed image from a coarse pattern portion by arranging a sub resolution assist feature with not-more-than the limit of resolution between mask patterns on a photomask.
With this technology, an exposure mask (photomask) comprises glass and a plurality of mask patterns containing chromium (Cr) coated to a glass surface. The mask pattern includes a main pattern subjected to deformation to offset the optical proximity effect due to light and etching based on a design pattern. Further, the mask pattern is arranged under a rule preset in accordance with the distance between the main patterns, and further includes an assist pattern for assisting the resolution of the main pattern, with a width of not-more-than the limit of resolution. Therefore, the existence of the assist pattern allows an optical image of the coarse main pattern to be close to an optical image of the main pattern with high density, thereby ensuring the depth of focus of the optical image of the coarse main pattern portion, approximate to that of the optical image of the main pattern with high density (e.g., Patent Document 1).
However, even in the case of creating the assist pattern under the preset rule, a high depth of focus cannot be obtained for all main patterns.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-100390